\documentclass{standalone}

\usepackage{amsmath}
\usepackage{xcolor}
\usepackage{graphicx}

\definecolor{JuliaRed}{RGB}{203, 60, 51}
\definecolor{JuliaViolet}{RGB}{149, 88, 178}
\definecolor{JuliaGreen}{RGB}{56, 152, 38}

\begin{document}
\nopagecolor
\resizebox{5cm}{!}{%
\(\boxed{\mathrm{\color{JuliaGreen}{prox}}_{\color{JuliaRed}{\gamma} \color{JuliaViolet}{f}}}\)%
}
\end{document}
